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LM7481: Reverse current blockin problem

Sahin Bozkurt
Prodigy 80 points
Part Number: LM7481
Other Parts Discussed in Thread: LM74800-Q1,

Dear TI team,

In my design i am using LM7481 as Oring circuit and reverse current blocker. My circuit has a battery which is activated during main power supply failure. In my tests, i am realizing power failure by cutting the 28V main input using a switch and the power goes down almost in 400us enabling battery. But, when battery is enabled, Q1 gate-source voltage goes low in almost 1.5ms resulting in reverse current flow into main power supply and disabling battery. As result my power supply is interrrupted and the test has failed. From the power management forum i have seen that LM7481 detects reverse current by measuring C pin to A pin voltage level and if it exceeds -5.5mV threshold the ic shuts down Q1. But until then it allows a reverse current flow. I am using a mosfet which has 2.2mohm on resistance. As i understood, it is needed almost 2.5A reverse current before the mosfet is turned off to block reverse current? 

In my design i am using the reference circuit in the datasheet page-16, figure 9-1, except VOUT2 output.

Could you please give me information about this reverse current property more detailed? And also could you please justify my findings about threshold for fast reverse current blocking voltage which is needed for Q1 to be shut down? 

  • 0
    TI__Mastermind 49295 points

    Hi Sahin,

    The understanding is correct, you need ~2.5A of reverse current to trigger internal reverse current comparator. LM7481-Q1 has ON/OFF gate control because of which DC reverse current flow is expected. You can consider LM74800-Q1 which ensure 0A DC reverse current flow. It uses regulated gate control method.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 80 points

    Hi Shiven,

    Thanks for your quick response to my question. But as you said it is needed 2.5A reverse current to turn off mosfet. But this reverse current condition causes my battery voltage to be seen from my main supply. This situation may damage my main supply because my battery voltage rises very fast. And as i have mentioned in my first question, since mosfet cannot turn-off immediately, it causes my circuit to sense that the main power supply has come back and it should operate in normal operation conditions. Is there any way you can recommend which will help me to turn off mosfet really quick? 

    Or is there any possible way to be able to turn-off mosfet without waiting it to sense -5.5mV reverse voltage?

  • 0 in reply to Sahin Bozkurt
    TI__Mastermind 49295 points

    Hi Sahin,

    You can either use a higher RDS FET or change controller to LM74800-Q1 which has linear regulation gate control and you will not face this issue.

    Regards,

    Shiven Dhir